1. Field of the Invention:
This invention relates generally to an arrangement for controlling the acceleration of a turbine system from start-up to synchronous speed, and more specifically, this invention relates to an arrangement for controlling the acceleration of a steam turbine system having bypass lines across the high pressure turbine and across the intermediate and low pressure turbines, such as in a turbine system interfaced with a high temperature gas cooled reactor (HTGR).
2. Description of the Prior Art:
In a conventional steam turbine system where all of the steam flow passes through all the turbine elements in the system (except for the extraction flow), control of acceleration of the turbine-generator rotor upon start-up must be accomplished by adjustment of the throttle valve or the control valve, which regulates flow through the high pressure, intermediate and low pressure turbine elements. The reheat stop and interceptor valves are normally kept wide open. Since all of the steam flow passes through all the turbine stages, the pressure in the reheater is, thus, dictated by the flow through the throttle and control valves. Since the reheat stop and interceptor valves are wide open, the high, intermediate and low pressure turbine torques are all given by the positioning of the throttle valve or the control valve, whichever is being utilized for startup control.
In some turbine systems, bypass lines are provided around the individual turbines for use during start-up of the system. Such a bypass arrangement for starting is generally referred to as a "European" type bypass arrangement. A similar type of bypass arrangement, in which a first bypass line is connected across the high pressure turbine and a second bypass line is connected across the intermediate and low pressure turbines, is utilized when a steam turbine is interfaced with a "once through" steam supply system, such as a high temperature gas cooled reactor (HTGR). In a turbine interfaced with an HTGR system, a drive arrangement for the cooling gas (e.g., helium circulators) is provided by additional steam turbines in the steam flow path of the main turbine system. In order to remove the heat generated in the reactor core, these turbines must be driven at all times, even when the main turbine system is not providing a power output. Accordingly, a minimal amount of steam flow must be provided by an auxiliary boiler system before reactor start-up. The bypass lines across the high pressure turbine and across the intermediate and low pressure turbines provide a steam path for this initial flow. Normally, the minimum reactor flow is 25%, due to steam generator stability problems. This flow will bypass the main turbine, if the main turbine is shut down.
With a steam turbine having a "European" type bypass arrangement, or a steam turbine interfaced with an HTGR, acceleration of the turbine system is achieved by diverting a portion of the flow from the bypass lines to the turbines. The type of control utilized in conventional systems cannot be utilized with this type of system, since the reheater is pressurized (25% of rated flow) before steam turbine start-up, and thus the reheat stop and interceptor valves cannot be fully opened during start-up. Further, to achieve acceleration control by just modulating the throttle and control valves is not feasible, since a certain amount of steam flow is required to the intermediate and low pressure turbines. The steam flow to the intermediate and low pressure turbines is required for exactly opposite reasons. During start-up of the low pressure turbine, the "windmilling" of the turbine blades results in a large heat buildup in these blades, and thus the steam is required for cooling purposes. In the intermediate turbine, on the other hand, it is necessary to heat the metal of the turbine blades above a certain critical temperature prior to loading, and thus the steam through the intermediate pressure turbine provides a warming or heating function. Therefore, the interceptor valve must be utilized to control steam flow to the intermediate and low pressure stages during start-up. As only a small flow is required, this control is difficult for the interceptor valve, as this valve is designed for control of much greater flow rates.